Angels praise God

Psalm 103:20-21 Praise the LORD, you his angels, you mighty ones who do his bidding, who obey his word. Praise the LORD, all his heavenly hosts, you his servants who do his will.

(See also Psalm 148:2 Praise him, all his angels; praise him, all his heavenly hosts.)

Angels have good reason to praise God - For their creation by God, for their preservation, for their happiness, in which they continue as they are elect of God in Christ.

They are always employed in the work of praising the Lord nor are they in the least backward in it, nor remiss. That is not why they are addressed as they are. Rather the design of the psalmist is only to show how great and good the Lord is and that angels, as great as they are, are under an obligation to praise him, and are bound to do so. He also intends to stir himself and others to this work, from such a consideration - that such noble creatures praise God.

C H Spurgeon

Finding his work of praise growing upon his hands, he calls upon "the firstborn sons of light" to speak the praises of the Lord, as well they may, for as Milton says, they best can tell. Dwelling nearer to that prepared throne than we as yet have leave to climb, they see in nearer vision the glory which we would adore. To them is given an exceeding might of intellect, and voice, and force which they delight to use in sacred services for him; let them now turn all their strength into that solemn song which we would send up to the third heaven. To him who gave angelic strength let all angelic strength be given. They are his angels, and therefore they are not loath to ring out his praises.

Angels will be judged one day - by believers

Paul says

1 Cor 6:3 Do you not know that we will judge angels? How much more the things of this life!

There is some mystery here but at the very least we can say that some angels will be judged and that believers will have some part in that judgement. No doubt that part will be a confirmatory one rather than anything more reliant on our own decisions. The fallen angels will be condemned to hell and the good angels will be confirmed for heaven. No doubt the focus lies on the fallen angels.

Angels revealed and announced the resurrection of Christ

Matthew 28:2-7 There was a violent earthquake, for an angel of the Lord came down from heaven and, going to the tomb, rolled back the stone and sat on it. His appearance was like lightning, and his clothes were white as snow. The guards were so afraid of him that they shook and became like dead men. The angel said to the women, "Do not be afraid, for I know that you are looking for Jesus, who was crucified. He is not here; he has risen, just as he said. Come and see the place where he lay. Then go quickly and tell his disciples: 'He has risen from the dead and is going ahead of you into Galilee. There you will see him.' Now I have told you."

Luke 24:2-7 They found the stone rolled away from the tomb, but when they entered, they did not find the body of the Lord Jesus. While they were wondering about this, suddenly two men in clothes that gleamed like lightning stood beside them. In their fright the women bowed down with their faces to the ground, but the men said to them, "Why do you look for the living among the dead? He is not here; he has risen! Remember how he told you, while he was still with you in Galilee: 'The Son of Man must be delivered over to the hands of sinners, be crucified and on the third day be raised again.'"

Angels - their negative work

1. Striking the Sodomites with blindness Genesis 19:11 Then they struck the men who were at the door of the house, young and old, with blindness so that they could not find the door.

2. Striking dead rebels in the wilderness in Moses' time 1 Corinthians 10:10 And do not grumble, as some of them did - and were killed by the destroying angel.

3. Willingness to slay Balaam by sword Numbers 22:31 Then the LORD opened Balaam's eyes, and he saw the angel of the LORD standing in the road with his sword drawn. So he bowed low and fell facedown. 

4. Driving out the Canaanites Exodus 33:2 I will send an angel before you and drive out the Canaanites, Amorites, Hittites, Perizzites, Hivites and Jebusites. 

5. Beginning to destroy the Israelites in David's time 1 Chronicles 21:15 And God sent an angel to destroy Jerusalem. But as the angel was doing so, the LORD saw it and relented concerning the disaster and said to the angel who was destroying the people, "Enough! Withdraw your hand." The angel of the LORD was then standing at the threshing floor of Araunah the Jebusite. (See 2 Samuel 24:16)

6. Slaughtering the Assyrians 2 Kings 19:35 That night the angel of the LORD went out and put to death a hundred and eighty-five thousand in the Assyrian camp. When the people got up the next morning - there were all the dead bodies! (See 2 Chronicles 32:21, Isaiah 37:36)

7. Striking Zechariah mute Luke 1:19, 20 The angel said to him, "I am Gabriel. I stand in the presence of God, and I have been sent to speak to you and to tell you this good news. And now you will be silent and not able to speak until the day this happens, because you did not believe my words, which will come true at their appointed time." 

8. Striking Herod dead Acts 12:23 Immediately, because Herod did not give praise to God, an angel of the Lord struck him down, and he was eaten by worms and died.

9. More generally Psalm 35:5, 6 May they be like chaff before the wind, with the angel of the LORD driving them away; may their path be dark and slippery, with the angel of the LORD pursuing them.

10. Various judgements Revelation 8;5 etc Then the angel took the censer, filled it with fire from the altar, and hurled it on the earth; and there came peals of thunder, rumblings, flashes of lightning and an earthquake. (See also Revelation 16, etc).